Be aware that August 2015 CU for SharePoint 2013 contains the required changes for SharePoint 2013 to enable this feature. The hybrid search functionality itself will be available as a preview in Microsoft Office 365 starting on September 7, 2015.

I have setup an Hybrid Search environment on SharePoint 2013 and SPO. Currently my Search Center is on On-Prem. I am getting both On-Prem and SPO docs on On-Prem search result page. All the settings and search result looks good.
However i am not able to Preview PDF docs coming from SPO. Preview is working for all MS office docs from SPO. I have tried to change the On-Prem search result type for PDF files to consume Word Item as display template but still not working. My issue is only with PDF preview not working from SPO files on Serach center which is on On-Prem. Can any one give valuable input to resolve this issue ?